High pressure is anything over 80 psi..
In many areas the water meters have integral check valves...
If your region has a code requiring expansion tanks in every residence, I would suspect that may be the case...
In any case, if code requires it generally there is a good reason....

What do you mean by " pressure tank" ? lf you mean an expansion tank, the answer is if code requires it , you need it. Not all areas require it, but most WH manufacturers recommend it. Its purpose is to absorb TEMPORARY pressure increase cause by expansion of water in a closed system while burner is running. It has NOTHING to do with the street pressure, because it will not change whatever the static pressure is inside your house.

Homes that "never needed it before" ....could be because the TP on the old WH was crudded shut! And it could be because newer water heaters tend to have higher BTU than old ones, and that faster recovery raises an expansion issue that was not noticed before.

Here, a new water heater install is required to be brought up to meet ALL current codes. This means installing earthquake straps ( not flimsy old ones, but straps certified by the California State Architect). Piping the TP discharge outside the dwelling. Fixing any flue issues which do not comply with building codes and National Fuel Gas Code. Correcting any combustion air supply issues. These things all cost money.

Water districts have been changing out water meters and some of the new ones have check valves.

My water heater failed months after the meter was replaced. Twice.
The first time, I replaced the leaking parts on top of the tank, put in a drain pan while I as at it.
One month later, the heater had failed at the welds.
Thank God, I had put in the drain pan.
The new heater went in, and this time I added the expansion tank, that was about 10 years ago.
I like the expansion tank, it feels like there is more water in the system, it's not so noticeable now when someone uses water while I'm in the shower.

It's code to have these when the system is closed, either by check valve or by a pressure reducing valve.

Her city water supply is redoing all the supply meters in the city. The new systems all have a check valve to eliminate for all time any chances of back flow from a residence to the city water supply.

With this new check valve in place, there is no possible relief for expansion of water in a house system. The resulting pressure fluctuation can only be released thru the expansion relief valve in your water heater. They are typically designed for an occasional use situation, and will probably fail with repeated use from demand.

We hadda put 'er in. You can argue that if water enters an empty heater, 50 deg. probably, heated to 125 deg., will expand about two cups worth. If any one turns on any hot water valve during this process, expansion is accomplished. But the expansion tank is allowing for the remotely possible extreme--but still possible.

My guess is the city either has check valves in the meters or is planning on them. This would create a closed system which requires an expansion tank. I respectively disagree with Plumber Mark regarding 90 psi. You don't need nearly that much pressure. I have mine reduced to 50 and that works very well. I'm not to to mess with it just to prove a point, but I would not be at all surprised if 35 or 40 psi would work just as well. Remember, you have to know the difference between pressure and flow volume. If your pipes are corroded galvanized pipe, then you might need more pressure to force more flow. Clean pipes will have sufficient flow at a lower pressure.

As the others stated if the local code state Thermal Expansion Tanks are required it is what it is! There is probably something installed by them (water co.) that creates a closed system. As the others have stated many meters have check valves!

Water districts have been changing out water meters and some of the new ones have check valves.

Click to expand...

I don't know if mine has a check valve in it but the city did change it out about a month ago. The new one has a transponder/anntenna or something like that on it so they can read it without getting out of car.

If they told you you needed an expansion tank, then it probably does have a check valve in it. In that situation, after running a long shower, it is not uncommon for the WH to cause the pressure to exceed the safety valve, and dump some water out. You want to avoid that, as that high pressure can burst a washing machine hose or possibly one to the icemaker, etc. It can create premature death to your WH tank, too.

You may or may not "NEED" it, because that depends on your house's particular plumbing system. But, if the code says you must have it, that supercedes whether you need it or not, and you have to install it.